Quantitative, chemical measurements

The acronym for chemical process quantitative risk analysis. It is the process of hazard identification followed by numerical evaluation of incident consequences and frequencies, and their combination into an overall measure of risk when applied to the chemical process industry. It is particularly applied to episodic events. It differs from, but is related to, a probabilistic risk analysis (PRA), a quantitative tool used in the nuclear industry... 

Guidelines for Chemical Process Quantitative Risk Analysis (CPQRA Guidelines) builds on the earlier work to show the engineer how to make quantitative estimates of the risk of the hazards identified. The quantitative estimates can identify the major contributors to risk. They can also help to define the most effective ways to a safer process by indicating relative risk reduction from proposed alternate process safeguards and measures. 

Chemical Process Quantitative Risk Analysis(CPQRA) The numerical evaluation of both incident consequences and probabilities or frequencies and their combination into an overall measure of risk. 

Potential measurement This technique has provided valuable information as to the condition of passive/active materials, particularly in the chemical industryAlthough quantitative weight loss measurements are not obtained, measurements can be on-line and more importantly, can be monitored using the actual plant material in situ) as a sensor. 

Measurement of exposure can be made by determining levels of toxic chemicals in human serum or tissue if the chemicals of concern persist in tissue or if the exposure is recent. For most situations, neither of these conditions is met. As a result, most assessments of exposure depend primarily on chemical measurements in environmental media coupled with semi-quantitative assessments of environmental pathways. However, when measurements in human tissue are possible, valuable exposure information can be obtained, subject to the same limitations cited above for environmental measurement methodology. Interpretation of tissue concentration data is dependent on knowledge of the absorption, excretion, metabolism, and tissue specificity characteristics for the chemical under study. The toxic hazard posed by a particular chemical will depend critically upon the concentration achieved at particular target organ sites. This, in turn, depends upon rates of absorption, transport, and metabolic alteration. Metabolic alterations can involve either partial inactivation of toxic material or conversion to chemicals with increased or differing toxic properties. 

Risk can be measured and expressed in a number of ways. CCPS s Guidelines for Chemical Process Quantitative Risk Analysis (Ref. 4) identifies three main categories of risk measure Risk Indices, Individual Risk, and Societal Risk. 

In addition to the direct analysis of a sample for its quantitative and/or qualitative composition, HS-GC can be used for physico-chemical measurements, such as the determination of vapour pressures. 

Whatever the reason for obtaining it, a chemical measurement has a certain importance since decisions based upon it will very often need to be made. These decisions may well have implications for the health or livelihood of millions of people. In addition, with the increasing liberalization of world trade, there is pressure to eliminate the replication of effort in testing products moving across national frontiers. This means that quantitative analytical results should be acceptable to all potential users, whether they be inside or outside the organization or country generating them. 

The fundamental unit in chemical measurement is the mole - amount of substance. A mole is the amount of a substance that contains as many atoms, molecules, ions or other elementary units as the number of atoms in 0.012 kg of carbon 12 (12C). It is the only dimensionless SI unit. In practical terms, it is almost impossible to isolate a mole of pure substance. Substances with a purity of better than 99.9% are rare one exception is silver, which can be obtained with a purity of 99.9995% which is referred to as five nines silver . Another problem is that it is not always possible to isolate all of the analyte from the sample matrix, and the performance of the chemical measurement may be matrix-dependent - a given response to a certain amount of a chemical in isolation may be different from the response to the same amount of the chemical when other chemicals are present. If it is possible to isolate quantitatively all of the analyte of interest from the accompanying sample matrix, then a pure chemical substance may be used for calibration. The extent to which the analyte can be recovered from the sample matrix will have been determined as part of the method validation process (see Chapter 4, Section 4.6.3). 

The result of a quantitative chemical measurement is not an end in itself. It has a cost and therefore it always has a purpose. It may be used, for example, in checking products against specifications or legal limits, to determine the yield of a reaction, or to estimate monetary value. 

Boron-11 n.m.r. spectroscopy is a very valuable means for the study of iminoboranes since it permits to distinguish between monomers and dimers. The former species, which contain three-coordinated boron, absorb at lower filed [chemical shift 6 about -40 ppm relative to external F3BO(C2H5)2 ] whereas the latter contain four-coordinated boron and absorb at higher fields (5 about -4 ppm). Monomer-dimer equlibria can be measured quantitatively by this method 39K... 

Risk is defined as a measure of human injury, environmental damage, or economic loss in terms of both the incident likelihood (probability) and the magnitude of the loss or injury (consequence) (AICHE/CCPS, Guidelines for Chemical Process Quantitative Risk Analysis, 2d ed., American Institute of Chemical Engineers, New York, 2000, pp. 5-6). It is important that both likelihood and consequence be included in risk. For instance, seat belt use is based on a reduction in the consequences of an accident. However, many people argue against seat belts based on probabilities, which is an incorrect application of the risk concept. 

As an example, consider the measurement of the purity of a chemical by quantitative NMR, using the peak of an added CRM as an internal standard. The purity of the test material (Ptest) is given by... 

A variety of mathematical models can be used to establish appropriate relationships between instrumental responses and chemical measurands. Quantitative analysis in single-element atomic absorption spectroscopy is typically based on a single measured signal that is converted to concentration of the analyte of interest via the calibration line ... 

Thus, whereas chemists have historically expressed analyses mostly by mass per mass, or as convenient percentages, or by mass per volume, they could express their measurements in amount of a specific substance per mass (mole per kilogram) or per volume. In cases such as pure materials and gases, mole per mole can be used. A percentage statement, or one in parts per thousand, million, or billion, is possible, though not recommended. In the SI system, as originally visualized, such dimensionless numbers as results of measurements are not favored. The quantitative result of any measurement should be expressed by a number multiplied by the appropriate unit associated with the measured quantity. As is further discussed below, this original preference proposed for the International System does not fit well with much of current practice in chemical measurements. 

Not all chemical measurements are, or should be, traceable to the mole. We have seen instances where the unit of mass was the proper SI unit for a quantitative measurement of a material of unspecified entities. There are chemical measurements that are not, but probably should be, referred, and preferably be traceable, to the SI unit. Color is used either simply as a qualitative attribute not subject to a measurement, or it is measured quantitatively by some spectrometry, where it may inevitably be subject to high uncertainties from both the measurement itself as well as from theory, such as the Lambert-Beer Law, but well understood in relation to SI. 

The main source of conformational information for biopolymers are the easy-to-obtain chemical shifts that can be translated into dihedral restraints. In addition, for fully 13C labeled compounds, proton-driven spin diffusion between carbons [72] can be used to measure quantitatively distances between carbons. The CHHC experiment is the equivalent of the NOESY in solution that measures distances between protons by detecting the resonances of the attached carbons. While both techniques, proton-driven spin diffusion and CHHC experiment [73], allow for some variation in the distance as determined from cross-peak integrals, REDOR [74] experiments in selective labeled compounds measure very accurate distances by direct observation of the oscillation of a signal by the dipolar coupling. While the latter technique provides very accurate distances, it provides only one piece of information per sample. Therefore, the more powerful techniques proton-driven spin diffusion and CHHC have taken over when it comes to structure determination by ss-NMR of fully labeled ligands. 

In contrast to many chemical measurements, which involve homogeneous bulk solutions, electrochemical processes take place at the electrode-solution interface. The distinction between various electroanalytical techniques reflects the type of electrical signal used for the quantitation. The two principal types... 

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